The electric motor of known thread supplying devices is an asynchronous motor needing complicated control electronics due its design and shows an operation performance which needs to accept some compromises when driving the thread guiding element of the thread supplying device. Among known thread supplying devices there are variants differing in equipment for threading of a new thread or even for automatically re-threading. When manually threading a new thread with a threading needle a guiding track provided in a housing bracket may be used for the threading needle in order to bring the new thread towards the withdrawal area. In case of an automatic or semi-automatic pneumatic threading system a guiding track may be provided as the threading means in the housing bracket and for the air stream conveying the new thread. In both cases it is necessary to register the thread guiding element for the threading procedure spatially with the threading means. For that purpose in many cases the motor shaft is rotated so far while stopping the electric motor by the motor control that the thread guiding element exactly is aligned with the threading means when the motor has stopped. In this case the target rotational position of the motor shaft corresponding to the alignment between the thread guiding element and the threading means will be achieved without the undesirable effect that the motor shaft continues to rotate further in the previous direction of rotation or counter thereto.
It is has been known for a long time to use a permanent magnet motor as the electric motor of a thread supplying device. Due to the design of the permanent magnet motor the rotor shows the tendency even without current supply to stop in one of several stable relative rotational positions when the motor is controlled to stop. The reason is that there occur alternating stable and instable relative rotational positions of the rotor in a permanent magnet motor. If the rotor randomly stops in the region of an instable rotational position then the rotor by itself rotates further without external influences either in the previous direction of rotation or counter thereto in order to find a stable rotational position. The phenomenon of the permanent magnet motor results in problems in a thread supplying device being equipped with a threading means when the motor shaft has to stop in a determined target position in which the thread guiding element is intended to co-act with the threading means.
It is an object of the invention to provide an improved thread supplying device of the kind as disclosed allowing to reliably reproduce target rotational position needed for the threading procedure in stopped condition.
Equipping the thread supplying device with the permanent magnet motor results in the advantage that fewer compromises have to be accepted in view to the operational performance than in case of an asynchronous motor. The permanent magnet motor can be controlled more simply, is compact and can be manufactured for fair costs in series production. The permanent magnet motor has overall the important advantage that it easily can be integrated into computerised data transmission systems and signal transmission systems, respectively, of textile machines. The unavoidable effect of several stable and instable rotational positions of the rotor without current supply due to the design of the permanent magnet motor intentionally is used to retain the thread guiding element in the threading position relatively stably, because the co-operating component group consisting of the stator, the rotor of the permanent magnet motor, the motor shaft and the thread guiding element are mounted in the housing such that one of the stable relative rotational positions of the rotor is the target rotational position of the motor shaft and, consequently, the threading position of the thread guiding element. By this structural measure it is possible to position the thread guiding element during the stop phase of the permanent magnet motor correctly in the threading position and to safely retain it in the threading position without additional means. When preparing the concept of the thread supplying device under consideration of the stable and instable relative rotational positions of the rotor it is necessary to exclude random motor mounting positions but to ensure a predetermined motor mounting position when assembling and designing the thread supply device.
In view to simple manufacturing exclusively the rotational position of the stator in the housing is adapted to the target rotational position corresponding to the threading position of the thread guiding element, provided that there is a fixed interrelation between the motor shaft and the rotor and the thread guiding element. This means that the rotor is fixedly secured on the motor shaft and that thread guiding element is fixedly connected to the motor shaft.
There are, however, several possibilities to avoid the influence of the instable relative rotational positions of the rotor of the permanent magnet motor for the desired threading position of the thread guiding element. The already mentioned rotational positioning of the stator in the housing by means of a positioning device is the first possibility. Alternatively or additively, there even may be provided a rotational positioning device between the rotor and the motor shaft in order to achieve the desired threading position of the thread guiding element correlated to a stable relative rotational position of the rotor, and alternatively or additively even also a rotational positioning device between the thread guiding element and to the motor shaft. The finally selected possibility depends on given structural prerequisites in the thread supplying device.
Expediently, at least one engagement location is provided at the stator. At this engagement location an engaging element of the housing is brought into engagement in order to save the rotational position the stator in the housing such that one of the stable relative rotational positions correlates with the threading position of the thread guiding element.
In view to simple manufacturing the engagement location is formed as a recess or a longitudinal groove in the periphery of the stator. A projection of the housing then is brought into engagement with this engagement location. Expediently, then a locking screw is screwed in assuring the position of the stator.
For assembly reasons the engagement element ought to be provided at the lower side of the housing. In adaptation to that position of the engagement element in the housing the respective engagement location at the stator is placed at an angle of rotation of the rotor which corresponds to in the stable relative rotational position.
Since the rotor takes several stable relative rotational positions within a full revolution, it may be expedient to distribute a number of engagement locations around the circumference, the number corresponding to the number of pole pairs of the stator, and to selectively use one of them for positioning the stator by means of the engagement element.
The threading means may, as known per Se, be constituted by a guiding track in a housing bracket, the guiding track being provided either for a threading needle or for a threading air stream. In the target rotational position of the motor shaft the exit of a winding tube constituting the thread guiding element of the motor shaft will be radially aligned with the guiding track, as soon as the rotor of the permanent magnet motor after switching off the current has taken one of its predetermined stable rotational positions. The control electronics of a permanent magnet motor permanently uses information on the rotational angle of the rotor. This information may be used to move the rotor during a stop of the permanent motor by means of a motor stop routine into the stable relative rotational position correlated to the target rotational position of the motor shaft.
Alternatively or additively, even the position signal of a rotation position sensor of the thread supplying device may be used which provides a confirmation for the motor control that the thread guiding element has reached the threading position.